Midrasch zu Schemuel I 17:5

5) ("fins and scales") This tells me only of (fish with) an abundance of scales and fins, such as kiponoth. Whence do I derive the same for a fish that has only one fin and one scale? From (Vayikra 11:10) "All that do not have (lit.) a fin and a scale (kaskas)." R. Yehudah says: two scales. And even though there is no proof for this, there is an "intimation" for it, viz. (I Samuel 17:5): "and in armor of scales (kaskasim, plural) was he clothed." R. Yossi b. Dormaskita says: The leviathan is a clean fish, as it is written (Iyyov 41:7): "Its (the leviathan's) pride is the strength of shields" — these are its scales; (Iyyov 41:22): "Beneath it are sharp shards" — these are its fins.